The present invention relates to a cable joint between two medium voltage electric power cables, each cable having a conductor surrounded by a conductor insulation and a semi-conductive layer, said cable joint comprising:
a connector constituted by a bolted contact assembly having a tubular shape and being adapted for connecting together bared ends of said conductors engaged in said contact assembly and maintained therein by means of bolts; and
an insulating sleeve adapted for covering at least said connector.
Such a cable joint is already known in the art, e.g. from the UK Patent Application GB-A-2 254 739 entitled xe2x80x9cCable Jointxe2x80x9d by J. W. Weatherley et al. (Raychem Limited), first filed on Mar. 25, 1991. The connector used in this known cable joint is covered by a heat shrinkable insulating sleeve, generally known as joint-body. The heat shrinkable joint-body is positioned over the connector and caused to shrink by heating and, once the heating is removed, the joint-body remains in place. The retracted joint-body tends to follow the shape of the underlying connector and conductor insulation of each cable. However, there are remaining gaps at the location where the diameters change, i.e. between the conductor insulation and the contact assembly, as well as other gaps due to the irregular shape of this bolted contact assembly.
The difference in diameter between the conductor insulation and the contact assembly is mainly due to the fact that the bolted contact assembly generally covers a wide range of cross-sections, e.g. from 50 to 300 mm2, and is thus larger than the conductor insulation. Such a bolted contact assembly is for instance known in the art from the UK Patent Application GB-A-2 262 396 entitled xe2x80x9cConnector for elongate objectsxe2x80x9d by D. J. Hollick (B and H), first filed on Dec. 14, 1991. Additionally to the fact that the diameter of the bolted contact assembly is bigger than the diameter of the conductor insulation, it is further positioned eccentrically with respect to the conductor. This is due to the presence of the bolts which still increase the dimensional difference between conductor insulation and the connector. As a result, the insulating sleeve is not able to closely follow the contours of the conductor insulation and of the bolted contact surfaces, leaving a large air gap in front of the contact, right at the electrically speaking most critical point of the joint. This is a problem, especially for applications above 10 or 15 kVolt.
In the known cable joint, these gaps are reduced by wrapping the connector with an amount of insulating mastic.
However, it is difficult to determine how much filling has to be put into the gaps under the insulating sleeve. Furthermore, the mastic used to compensate for the gaps may flow away, e.g. during temperature cycles. The insulating sleeve, being relatively rigid once retracted, might not compensate for this flow, resulting in the appearance of more gaps.
Another problem with the known cable joint is that high electrical fields exist at the edges of the connector. These local electrical stresses may result in partial discharges and in product failure.
An object of the present invention is to provide a cable joint of the above known type but which is better adapted to sustain local electric stresses, especially at the intersection between the connector and the conductor insulation, while being also better adapted to take into account the diameter differences of the items under the insulating sleeve.
According to the invention, this other object is achieved due to the fact that said cable joint is further provided with two molded caps of semi-conductive rubber covering part of the conductor insulation of the two cables and joining each other over said connector, each of said caps having an internal shape adapted for being engaged over the connector and over the conductor insulation and for mating the change of dimension between said connector and said conductor insulation.
In this way, a relatively cheap and efficient solution is provided to compensate for the gap resulting from the difference in diameter between the conductor insulation and the connector, as well as for the eccentric position of the latter. Furthermore, the semi-conductive material used for making the caps provides a better distribution of the electrical fields around the connector and thus an electrically more reliable product than that of the known prior art.
Another characterizing embodiment of the present invention is that each of said molded caps has an external shape adapted for smoothing the transition from the external diameter of said conductor insulation to the external diameter of said connector by providing an external conical transition with a relatively slow slope.
This characteristic of the caps having an outer surface smoothing the sharp edges of the contact, e.g. the shearing bolts, is also beneficial to reduce local electric stresses.
Also another characterizing embodiment of the present invention is that said two molded caps overlap each other in the middle of the connector.
In this way, the molded caps are usable for different lengths of bolted contacts. The overlapping further ensures that the underlying connector is not exposed.
Still another characterizing embodiment of the present invention is that said insulating sleeve is a cold shrinkable joint-body adapted, when retracted, to cover said connector covered by said molded caps, the conductor insulation and part of the semi-conductive layer of both said cables.
The heat shrinkable joint-body known from the above first mentioned UK Patent Application is caused to retract by heating. In this condition the material becomes elastic and is able to exert pressure on the underlying filling, but once cooled down, it becomes rigid again and will not compensate for flowing of the filling material. On the other hand, a cold shrinkable joint-body is positioned by removing the support tube that maintains it in an expanded status. When retracted over the connector, the cold shrinkable joint-body exerts a pressure on the underlying layers. This pressure is maintained for ever, without any heating, and all the underlying items, including any filling mastic, are so maintained in place.
Yet another characterizing embodiment of the present invention is that said cold shrinkable joint-body comprises an inner layer of insulating mastic, an intermediate layer of insulating material and an outer layer of semi-conductive material.
This mastic is conformed by the shrinking joint body and fills any possible remaining irregularities over the connector and conductor insulation area.
The present invention also relates to a method of forming a joint between two medium voltage electric power cables, each cable having a conductor surrounded by a conductor insulation itself surrounded by a semi-conductive layer, the end of the semi-conductive layer being cut-back to expose a bared end of said conductor insulation and the end of the conductor insulation being removed to expose a bared end of said conductor.
In this method:
a molded cap, having a first section with a first internal diameter adapted for being engaged over said conductor insulation and a second section with a second internal diameter adapted for being engaged over a connector, is engaged over each of said conductor insulations;
the bared conductor ends of the two cables are connected together by means of said connector constituted by a bolted contact assembly having a tubular shape, said conductor ends being maintained therein by means of bolts; and
the molded caps are slid over said connector until they joint each other with their second section covering said connector, said caps having, between their first and their second section, an internal shape adapted for mating the change in dimension between said first and said second internal diameter.
In this way, a cable joint is obtained whereof the internal shape of the caps closely follows the contours of the underlying conductor insulation and external surface of the bolted contact assembly, reducing and electrically neutralizing thereby any possible air gap between these items.
Another characterizing embodiment of the present method is that each of said molded caps has an external shape adapted for smoothing the transition from the external diameter of said conductor insulation to the external diameter of said connector by providing an external conical transition with a relatively slow slope.
A conductor connector area with a smoother outer surface is so obtained.
The present invention further also relates to a tubular assembly for a cable joint adapted for covering a connector constituted by a bolted contact assembly adapted for connecting together bared ends of conductors of two medium voltage electric power cables.
A further object of the present invention is to provide a tubular assembly able to improve the quality of the cable joint.
According to the invention, this object is achieved due to the fact that said tubular assembly is constituted by two molded caps made of semi-conductive rubber and joining each other over the connector, and that each of said molded caps has an internal shape adapted for being engaged over said connector and the insulation of said conductor and for mating the change of shape between said connector and said conductor insulation.
In this way, the internal shape of the caps closely follows the contours of the underlying conductor insulation and the external surface of the bolted contact assembly, reducing and electrically neutralizing thereby any possible air gap between these items.
Another characterizing embodiment of the present invention is that each of said molded caps has an external shape adapted for smoothing the transition from the external diameter of a conductor insulation of said cable to the external diameter of said connector by providing an external conical transition with a relatively slow slope.
In this way, the connector has a smoother outer surface with all the advantages thereof, e.g. reducing the electrical stresses and allowing the joint body to adapt better to the underlying surface.
Further characterizing embodiments of the present cable joint using a semi-conductive tubular assembly, method for obtaining such a cable joint and tubular assembly for this type of cable joint are mentioned in the appended claims.